The present disclosure generally relates to subterranean operations. More particularly, the present disclosure relates to improved cementing plugs and methods of using these cementing plugs in subterranean wells.
During the drilling and construction of subterranean wells, it may be desirable to introduce casing strings (“casing”) into the wellbore. To stabilize the casing, a cement slurry is often pumped downwardly through the casing, and then upwardly into the annulus between the casing and the walls of the wellbore. Once the cement sets, it holds the casing in place, facilitating performance of subterranean operations.
Prior to the introduction of the cement slurry into the casing, the casing may contain a drilling fluid or other servicing fluids that may contaminate the cement slurry. To prevent this contamination, a cementing plug, often referred to as a “bottom” plug, may be placed into the casing ahead of the cement slurry as a boundary between the two. The plug may perform other functions as well, such as wiping fluid from the inner surface of the casing as it travels through the casing, which may further reduce the risk of contamination. After the bottom plug reaches the landing collar, a part of the plug body may rupture to allow the cement slurry to pass through.
Similarly, after the desired quantity of cement slurry is placed into the wellbore, a displacement fluid is commonly used to force the cement into the desired location. To prevent contamination of the cement slurry by the displacement fluid, a “top” cementing plug (“top plug”) may be introduced at the interface between the cement slurry and the displacement fluid. This top plug also wipes cement slurry from the inner surfaces of the casing as the displacement fluid is pumped downwardly into the casing. Sometimes a third plug may be used, for example, to perform functions such as preliminarily calibrating the internal volume of the casing to determine the amount of displacement fluid required, or to separate a second fluid ahead of the cement slurry (e.g., where a preceding plug may separate a drilling mud from a cement spacer fluid, the third plug may be used to separate the cement spacer fluid from the cement slurry).
A float valve or float collar is commonly used above the landing collar to prevent the cement from flowing back into the inside of the casing. When the bottom plug arrives at the float valve, fluid flow through the float valve is stopped. Continued pumping results in a pressure increase in the fluids in the casing, which indicates that the leading edge of the cement composition has reached the float valve.
Operations personnel then increase the pump pressure to rupture a frangible device within the bottom plug. Said frangible device may be in the form of a pressure sensitive disc, rupturable elastomeric diaphragm, or detachable plug (stopper) portion which may or may not remain contained within the bottom plug. After the frangible device has failed, the cement composition flows through the bottom plug, float valve and into the annulus. When the top plug contacts the bottom plug which had previously contacted the float valve, fluid flow is again interrupted, and the resulting pressure increase indicates that all of the cement composition has passed through the float valve.
The cementing plug also wipes drilling fluid from the inner surface of the pipe string as it travels through the pipe string, thereby preventing contamination of the cement slurry by the drilling fluid as it is pumped downhole. Once placed in the annular space, the cement composition is permitted to set therein, thereby forming an annular sheath of hardened, substantially impermeable cement therein that substantially supports and positions the casing in the wellbore and bonds the exterior surface of the casing to the interior wall of the wellbore.
A cementing plug typically has a nose on its downhole end to help it land and engage into the landing collar at the bottom of the wellbore. Conventional cementing plugs travel downhole with a nose extended toward the bottom of the borehole. However, the extended nose causes the center of mass of the cementing plug to be offset. The cementing plug, therefore, is not balanced while traveling downhole. Additionally, the nose may get stuck to the sides of the casing or other protrusions or irregularities in its path. With the nose stuck, the cementing plug may not be able to travel downhole. As the pressure from the fluid above the cementing plug increases, the fluid may eventually bypass the cementing plug and cause undesirable contamination.